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FreeBSD-SA-13:14.openssh                                    Security Advisory
                                                          The FreeBSD Project

Topic:          OpenSSH AES-GCM memory corruption vulnerability

Category:       contrib
Module:         openssh
Announced:      2013-11-19
Revised:        2013-11-28
Affects:        FreeBSD 10.0-BETA
Corrected:      2013-11-19 09:35:20 UTC (stable/10, 10.0-STABLE)
                2013-11-19 09:35:20 UTC (stable/10, 10.0-BETA3-p1)
                2013-11-19 09:35:20 UTC (stable/10, 10.0-BETA2-p1)
                2013-11-19 09:35:20 UTC (stable/10, 10.0-BETA1-p2)
CVE Name:       CVE-2013-4548

For general information regarding FreeBSD Security Advisories,
including descriptions of the fields above, security branches, and the
following sections, please visit <URL:http://security.FreeBSD.org/>.

0.   Revision History

v1.0  2013-11-19 Initial release.
v1.1  2013-11-28 Corrected path to sshd_config.

I.   Background

OpenSSH is an implementation of the SSH protocol suite, providing an
encrypted and authenticated transport for a variety of services,
including remote shell access.

AES-GCM (Galois/Counter Mode) is a mode of operation for AES block
cipher that combines the counter mode of encryption with the Galois
mode of authentication which can offer throughput rates for state of
the art, high speed communication channels.

OpenSSH supports the AES-GCM algorithm as specified in RFC 5647.

II.  Problem Description

A memory corruption vulnerability exists in the post-authentication sshd
process when an AES-GCM cipher (aes128-gcm@openssh.com or
aes256-gcm@openssh.com) is selected during key exchange.

III. Impact

If exploited, this vulnerability might permit code execution with the
privileges of the authenticated user, thereby allowing a malicious
user with valid credentials to bypass shell or command restrictions
placed on their account.

IV.  Workaround

Disable AES-GCM in the server configuration. This can be accomplished by
adding the following /etc/ssh/sshd_config option, which will disable
AES-GCM while leaving other ciphers active:

Ciphers aes128-ctr,aes192-ctr,aes256-ctr,aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,aes256-cbc

Systems not running the OpenSSH server daemon (sshd) are not affected.

V.   Solution

Perform one of the following:

1) Upgrade your vulnerable system to a supported FreeBSD stable or
release / security branch (releng) dated after the correction date.

2) To update your vulnerable system via a source code patch:

The following patches have been verified to apply to the applicable
FreeBSD release branches.

a) Download the relevant patch from the location below, and verify the
detached PGP signature using your PGP utility.

# fetch http://security.FreeBSD.org/patches/SA-13:14/openssh.patch
# fetch http://security.FreeBSD.org/patches/SA-13:14/openssh.patch.asc
# gpg --verify openssh.patch.asc

b) Execute the following commands as root:

# cd /usr/src
# patch < /path/to/patch

Recompile the operating system using buildworld and installworld as
described in <URL:http://www.FreeBSD.org/handbook/makeworld.html>.

Restart the sshd daemon, or reboot the system.

3) To update your vulnerable system via a binary patch:

Systems running a RELEASE version of FreeBSD on the i386 or amd64
platforms can be updated via the freebsd-update(8) utility:

# freebsd-update fetch
# freebsd-update install

VI.  Correction details

The following list contains the correction revision numbers for each
affected branch.

Branch/path                                                      Revision
- -------------------------------------------------------------------------
stable/10/                                                        r258335
- -------------------------------------------------------------------------

To see which files were modified by a particular revision, run the
following command, replacing NNNNNN with the revision number, on a
machine with Subversion installed:

# svn diff -cNNNNNN --summarize svn://svn.freebsd.org/base

Or visit the following URL, replacing NNNNNN with the revision number:

<URL:http://svnweb.freebsd.org/base?view=revision&revision=NNNNNN>

VII. References

<URL:http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2013-4548>

The latest revision of this advisory is available at
<URL:http://security.FreeBSD.org/advisories/FreeBSD-SA-13:14.openssh.asc>
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